Distance measurement system

ABSTRACT

Apparatus and a method using ultrasonic signals to determine the distance between the displacement of two points in space. An ultrasonic distance measurement unit (1) generates frequency modulated ultrasonic waves using binary shift keying. The signals are transmitted by the transmitting transducer (2) towards a reflector assembly (30), preferably comprised of two reflectors (31, 32). One of the reflectors (31) is smaller than the other reflector (32), and the smaller reflector (31) is positioned in front of the larger reflector (32). The reflected signals are received by the receiving transducer (3). The unit (1) phase digitizes and processes the received signal by recording time stamps of zero-crossings of the received signal. The speed of sound may be determined by a temperature measurement system (5) and/or by measuring the transit time difference between the two reflectors (31, 32) of the assembly (30). The distance between the unit (1) and the assembly (30) can then be determined from the speed of sound and the time taken for an ultrasonic signal to travel between the unit (1) and assembly (30). Displacement of the assembly (30) relative to the unit (1) can be determined by making distance measurements over a period of time. The present invention has a number of applications, including measurement of roof convergence in underground mining.

FIELD OF THE INVENTION

The present invention relates to a distance measurement system. Moreparticularly, it relates to apparatus and a method for signal processingultrasonic signals to determine the distance between and displacement oftwo points in space.

BACKGROUND ART

In various situations it is necessary to measure distances unobtrusivelybetween two points (i.e. there must be no physical members connectingthe points), or to measure changes in distances between the two points.It is often necessary for these measurements to be fully automated andto be made continuously or at regular intervals. One particular examplewhere regular unobtrusive distance and displacement measurements arerequired, is in the underground mining industry. Underground mine shaftsare prone to collapse, resulting in significant losses in productivityand possibly lives. Usually collapses of underground shafts are precededby the convergence of the shaft roof and floor. By detecting thisconvergence, it is possible to predict impending shaft collapses, andthus allows mining operators to attempt to avoid a possible disaster.

It is preferable that such a divergence detection system should have ahigh degree of accuracy, reliability and repeatability under conditionswhich are adverse while being unobtrusive and easily transportable.Existing measurement systems include extensiometers, wire-woundpotentiometers and laser interferometers.

Extensiometers and wire-wound potentiometers suffer from a number oflimitations. They rely on an obtrusive measurement technique resultingin errors due to mechanical disturbances arising from the general natureof mining operations. They are often used only as temporary apparatusand have limited resolution. For these reasons they cannot be used inmany areas where convergence measurements are required. Laserinterferometry suffers from the fact that a relatively clean environmentis necessary for correct operation. The environment within a mine is indirect conflict with this requirement. Furthermore, its cost make lasersystems very undesirable.

The present invention attempts to overcome one or more of the abovedisadvantages with the use of ultrasonic waves, ultrasound. Ultrasoundis comprised of travelling longitudinal mechanical waves at frequenciesabove those audible to the human ear, normally above twenty kilohertz.When travelling through air, the waves may be described in terms of thevariation of air pressure at a particular point. The pressure varieswith simple harmonic motion, firstly above and then below the averageatmospheric pressure at that point. The reflection of ultrasonic wavesfrom a plain surface is similar to the reflection of light from anon-ideal mirror. That is, the angle of reflection is approximatelyequal to the angle of incidence. This is especially true at highfrequencies. However, as the frequencies decrease, more defraction anddispersion take place.

Distance between and displacement of two points in space can be measuredusing ultrasound measurements. By comparing distance measurements atdifferent points in time, it is possible to detect relative movementbetween the points in space. In order to measure the distance betweenthe points, an ultrasonic toneburst can be projected from one of thepoints using an ultrasonic transducer, the toneburst is reflected by asuitable reflector at the second point and the toneburst then returns tothe point of transmission. The transit time of the pulse is proportionalto the total distance travelled. It is critical to accurately measurethe time between the transmission and reception of the toneburst, inorder to provide an accurate measurement of the distance between the twopoints.

In order to complete the distance measurement it is necessary to haveknowledge of the speed of the ultrasonic waves in air. This speed mayvary depending on temperature, air pressure, moisture content, etc. Ifit is known that the speed of sound does not change in the applicationof the UDMS, then a constant value for the speed of sound may be used.Alternatively, if the speed of sound does change, then a measurement ofthe current speed of sound is required.

FIGS. 1 and 2 illustrate known ultrasonic distance measurement systems(UDMS). FIG. 1 shows a UDMS unit 1 at a first point in space having atransmitting transducer 2, a receiving transducer 3 and a reflector 4 ata second point in space. The UDMS use known methods to determine thetime required for an ultrasonic signal to travel the unknown distance d1between the two points in space. If it is known that the speed of soundis variable over time around the UDMS unit 1, the UDMS 1 may also beprovided with a subsystem 5 for measuring the current temperature fromwhich it is possible to determine the speed of sound. Using the transittime period required for a signal to travel between the two points and afixed or measured value for the speed of sound, it is possible tocalculate the distance d1 between the two points.

FIG. 2 illustrates a UDMS 1 having a subsystem 5 comprised oftransmitting transducer 6 and receiving transducer 7 at a known distanced2 from each other. From a measurement of the transit time over theknown distance d2 it is possible to calculate the speed of sound. From ameasurement of the transit time between the UDMS 1 and the reflector 4,and the calculated speed of sound, it is possible to determine thedistance d1 between the two points.

The intensity of an ultrasonic wave in air attenuates at approximatelyinverse parabolic function against distance travelled. Furthermore, theattenuation becomes more rapid at higher frequencies. It is for thisreason that ultrasonic ranging systems use frequencies in the range 20to 200 kilohertz when measurements greater than a few meters arerequired. These frequencies correspond to wave lengths of 70 millimetersto 1.7 millimeters, respectively, in air. At this point it should alsobe noted that the spacial intensity distribution is a direct function ofthe frequency being transmitted. Higher frequencies result in areasonably directed output but suffer from rapid attenuation, whilstvery low frequencies result in less attenuation but a much morehemispherical distribution from an ultrasonic transducer. It is aproblem to find a particular operating frequency which has an acceptabledegree of attenuation and directionality for reliable operation of aUDMS over a wide range of distances.

Ultrasonic transducer convert electrical impulses to mechanicalultrasonic waves upon transmission, and vice versa when they arereceived by the transducers. The most commonly available ultrasonictransducers are of a piezoelectric or electrostatic type. As with mosttransducers, these are not ideal. One of the most significant problemswith transducers is that the transfer from electrical to mechanical, ormechanical to electrical waves, is not instantaneous.

The main problem with this is that the amplitude of the ultrasonic waveas it is transferred to air is not constant. Many cycles of the toneburst are required before the amplitude of the mechanical wave reachesits maximum. FIG. 3 illustrates a pulse 10 which is used to fire atransducer. The pulse 10 causes the transducer to produce an ultrasonicsignal 17. As shown, the ultrasonic signal 17 has a rise period 11before reaching its peak 12, and then a fall period 13 to a staticlevel. This phenomenon is due largely to the inertia of the diaphragmsin both the transmitting and receiving transducers. They cannot bebrought to their maximum displacement immediately upon application ofthe firing pulse. In the same manner, the toneburst does notautomatically cease upon the termination of electrical impulses. Theamplitude generally decays in an exponential manner.

The known ultrasonic ranging methods are inadequate for mining industryrequirements. The most common ranging method in pulse-echo rangingillustrated in FIG. 3. In this method a transmitting transducer is firedand the resulting ultrasonic signal 17 propagates away from thetransducer, is reflected from an object, and is then received by thereceiving transducer in an attenuated form 14. The time differencebetween the start of the signal at the transmitting transducer and thereception of the reflected signal is used to give an estimate of thedistance between the transducer and the object. The receiving transduceris triggered when a pulse is received which is greater than apredetermined threshold 15. This is usually achieved within the firstfew wavefronts of the ultrasonic signal. As described above, the longerthe distance travelled, the larger the attenuation of the receivedsignal. Since the amplitude of the received signal is compared to a setthreshold, and the received signal takes a number of cycles to build upto its maximum amplitude, the receiver will be triggered on differentwavefronts, depending on the distance and reflection properties of thereflecting object. This results in a significant error in themeasurement of the distance between the transducers and the object.

The variable gain method is a form of pulse echo ranging which partiallyreduces this error by varying the threshold or the gain of the receivingamplifier over time. By increasing the gain of the received signal overtime, the attenuation is counteracted. This method alleviates the aboveerror, but is not able to account for variations in intensity from othersources of amplitude variation, such as angle deviation errors andchanges in reflection properties.

Another known method is the modulated carrier method. In this method anultrasonic wave is continuously transmitted and is modulated by a lowerfrequency wave. The phase differences of the low frequency components atthe transmitter and receiver are examined and is directly proportionalto the straight line distance plus a constant. However, the method onlyhas a usable distance measurement range of one wavelength of the lowfrequency component. This is always less than one meter in magnitude.

The final known ranging method uses linear frequency modulation (LFM).In this method a chirp is transmitted. The frequency is varied in alinear sweep from low frequency to high frequency. This chirp isreceived and mixed with the transmitting waveform to produce arelatively constant, difference frequency. The difference frequency isproportional to the distance measured. The main problems involved withthis method is the limited bandwidth and non-linearity of thetransducers resulting in reduced accuracy.

Further errors in measurement can occur due to false triggering fromultrasonic noise, constructive and destructive interference between thesource and reflected ultrasonic waves, transducer bandwidth limitationsand changes in velocity of the ultrasonic waves due to air temperaturechanges. In many applications, such as mining environments, furthererrors occur due to temperature changes in the air over the distancebeing measured and over time. Any reliable method for compensating ameasurement system for changes in the speed of sound due to temperaturevariations may be used. A temperature transducer such as a thermistor orthermo-couple can be used. The correct speed can then be calculated forthe temperature of the environment when the measurement is made. Such aconfiguration was discussed ablove and shown in FIG. 1. Whilst thismethod is desirable in its simplicity, a significant problem arises ifthe air temperature is not homogeneous. That is, if the temperaturevaries significantly over the distance being measured, then significanterrors can result in the measured distance.

The present invention attempts to overcome one or more of the aboveproblems.

SUMMARY OF THE INVENTION

According to the present invention there is provided a method ofmeasuring the distance between a first point and a second point inspace, comprising the steps of:

generating and frequency modulating an ultrasonic signal using binaryfrequency shift keying,

transmitting the modulated signal from the first points towards thesecond point,

reflecting the transmitted signal at the second point back towards thefirst point,

receiving the reflected signal at the first point,

processing the received signal, including phase digitising the receivedsignal by recording zero-crossing transitions of the received signal,and

analysing the recorded transitions so as to determine the distancebetween the first point and the second point.

It is to be understood that the transmitted signal and received signalmay not be transmitted and received from exactly the same point inspace, but they are transmitted and received close to each other.

According to the present invention there further provides an apparatusfor measuring the distance between a first point and a second point inspace, comprising:

an ultrasonic signal generator at the first point for producing, in use,a frequency modulated ultrasonic signal using binary frequency shiftkeying and transmitting the modulated signal,

a reflector at the second point capable of reflecting the transmittedsignal,

a receiver capable of receiving the reflected signal,

signal processing means capable of processing and analysing the receivedsignal so as to calculate the distance between the first and the secondpoints in space by phase digitising the received signal, recordingzero-crossing transitions of the received signal and analysing therecorded transitions.

According to another aspect of the present invention there is provided adual reflector system comprising a first reflector and a secondreflector, the first reflector having a larger reflecting surface thanthe second reflector, and the second reflector being positioned in frontof the first reflector at a fixed reference distance from the firstreflector, wherein the reflector system is capable of reflecting a waveincident on the reflectors to produce two reflected waves travellingparallel to each other at a separation distance from each other, suchthat the speed of the waves is capable of being determined from theseparation distance and the reference distance.

DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the present invention will now be described, byway of example only, with reference to the accompanying drawings, inwhich:

FIGS. 1 and 2 illustrate a known ultrasonic distance measuring systems(UDMS),

FIG. 3 illustrates the known method of pulse-echo ranging,

FIGS. 4 to 7 are graphs illustrating a preferred embodiment of themethod according to the present invention,

FIGS. 8 & 9 illustrate preferred embodiments of the apparatus system inaccordance with a preferred embodiment of the present invention,

FIG. 10 illustrates a preferred embodiment of the reflector system ofthe present invention,

FIG. 11 is a block diagram of the apparatus according to a preferredembodiment of the present invention, and

FIG. 12 illustrates an application of the preferred embodiment.

DESCRIPTION OF PREFERRED EMBODIMENTS

The method of the preferred embodiment of the present invention employsfrequency modulation (FM) of the ultrasonic waveform with binary codingto produce a frequency shift keyed signal. When a distance between twopoints in space are to be measured, a frequency shift keyed signal istransmitted from one of the points, reflected off a reflector at thesecond point, and received at the first point. By measuring the transittime of the frequency shift keyed signal between the two points, it ispossible to determine the distance between the two points, since thetransit time is propostional to the distance travelled. Using thetransit time and the speed of the ultrasonic signal, it is possible tocalculate the distance.

Frequency shift keying (FSK) is a form of frequency modulation where thefrequency transmitted changes from a carrier frequency, F1,instantaneously to a second frequency, F2. In the example of thepreferred embodiment, the two frequencies are F1=40 kHz and F2=38 kHz.Note that these values are not critical, but should be stable for properoperation. The only limitation is in the range of frequencies that maybe used which is within the bandwidth of the transducers. Although theelectrical input changes instantaneously from frequency F1 to frequencyF2 at the transmitting transducer, the mechanical output, i.e. theultrasound waves, cannot instantaneously change, and so a transientoccurs resulting in a smooth transition of frequencies from F1 to F2.This normally has a duration of a few cycles. FIG. 4 is a graph of amodulated wave which has been received by a receiving transducer. ThisFSK wave has a slow transition from 40 kHz to 38 kHz in the middle ofthe graph.

In the UDMS of the preferred embodiment of the present invention the FSKsignal having the change in frequency is transmitted by a transducer atthe first point in space, the signal is reflected at the second point inspace, and the signal is received by a transducer at the first point inspace. The UDMS digitises the received signal by recording the timestamps of the negative-going zero-crossing transitions of the receivedsignal. That is, the received signal is digitised by storing a timestamp for every 360 degree phase change of the received signal. It is tobe appreciated that it is possible to record time stamps of thepositive-going zero-crossings instead of the negative-goingzero-crossings. This method eliminates any problems resulting fromamplitude variations of noise. FIG. 5 is a graph of recorded zero-goingnegative-transition events against time. Note that the transitionbetween the 40 kHz and 38 kHz frequencies cannot be clearly seen.

In order to extract the relevant data of the transition from the abovegraph, it is necessary to subtract a graph representing the carrier waveof 40 kHz. The result of this subtraction is a normalised graph which isillustrated in FIG. 6. It clearly shows the difference between thetransmitted frequencies. The slope of the 40 kHz section is 0, while theslope of the 38 kHz section is 2000, that is, the difference in slopeequals the difference between the two frequencies. Now, using thisgraph, a line of best fit is found using the 38 kHz data points only.This process is simplified by the fact that the slope of the 38 kHzsection is known. A second line of best fit is found for the 40 kHzsection. This line is horizontal, see FIG. 7. A least square errormethod may be used to find the lines of best fit for the data points.The magnitudes of the least square errors for the lines give anindication of the total error in the calculation of the distance betweenthe two points. The time of the transition between the two frequenciesis found from the intersection of the two straight lines. The distancebetween the first and second points can then be found by multiplying thecalculated transit time of the signal with the speed of sound in themedium. The speed of sound may be a known constant or may be calculatedfrom the temperature measurement subsystem or reference link(s) asdescribed below. This method of determining the distance between thepoints can be fully automated using a micro-processor system.

The analysis is based on phase information only. Thus, only steady stateinformation is relied upon. No information is used which is derived fromtransient waveforms. This is in direct contrast to previous methods ofultrasonic detection which only look at transient data. These methodsare open to errors resulting from variability of circuit elements,transducers and the characteristics of the transition medium. Incontract, the use of steady state information results in significantlyless sensitivity to errors due to the above sources, since the output ofthe receiver is virtually identical to the forcing function. The methodof the preferred embodiment is particularly advantageous since it isisolated from unwanted amplitude distortions, attenuation problems andenvironmental variations.

For the ultrasonic distance measurement system (UDMS) to work reliablyit is preferred that the system has means to process signals so thatfalse readings do not occur when vehicles, materials or personnelmomentarily obstruct the measurement path. In addition if the UDMS is tobe accurate in an industrial environment, it may also have an ability toreject noise that inevitably arises from electrical and mechanicalsources. To achieve such immunity the UDMS of the preferred embodimentmay employ signal processing techniques using a micro-processor.Furthermore, a micro-processor also enables the UDMS to interface withcommunications systems used throughout mines and industrial plant sothat remote monitoring may be easily achieved.

In the preferred embodiment of the present invention spurious errors indetermining the distance between the two points are eliminated byanalysing the received signal in various ways. Firstly, a singledistance calculation is done by sending a plurality of modulated signalsat regular time intervals. Each reflected signal is received and mustmeet predetermined noise, phase, and timing error bounds. For example, aplurality of pulses are transmitted at 100 millisecond intervals. Onlyonce 32 valid reflected pulses are received is the distance calculated.A further error reduction method uses to eliminate errors resulting frominterference or movement of people and equipment between the first andsecond points, involves the use of a "distance window". The plurality ofvalid readings obtained above are analyzed by placing a, for example, 2millimeter "window" over the readings. All distance readings outside the"window" are rejected as invalid readings. The final calculation of thedistance between the two points are then made using the average of thewindowed readings.

A system similar to the known ultrasonic distance measurement system ofFIG. 1 may be used in implementing the above methods. See FIG. 8. Thesingle reflector 21 provides adequate reflection of the ultrasonicsignals, while the main UDMS unit 20 contains the signal processingmeans to implement the method of the preferred embodiment of the presentinvention. The intelligence resulting from the use of micro-processortechnology may be used for local display of measurements by a variety ofmeans. For example in one preferred embodiment, different colouredlights can be illuminated to indicate the current convergence rage, e.g.Green--to indicate no significant convergence; Amber--to indicateconvergence in occurring; Red--to indicate significant convergence isoccurring and remedial actions is needed. The set point for indicationbeing programmed when installed. In another embodiment the UDMS may alsoincorporate an alpha-numeric display which can be interrogated todisplay the historical distance changes and rate of convergence. Eitherthe lights or numeric display can be used depending upon the location ofthe unit.

Due to the lightness and compactness of the UDMS, achieved by usingadvanced technology for signal processing, the unit can be easilyinstalled in all locations underground. The UDMS is designed to begenerally attached to the roof such as by attachment to a roof bolt,with the reflector on the floor or low on the rib, or wall. This is tominimise risk of damage to the main unit.

The UDMS may also be integrated with a computer system, either at theinstallation location or at a remote site. Distance and convergenceinformation can thus be communicated from the UDMS's micro-processor tothe computer system for further processing.

Using the single reflector is sufficient if the speed of sound isconstant over time. If it is not constant, then in accordance withanother preferred embodiment, a temperature measurement subsystem orreference link is used to determine the speed of sound. A knowntemperature measurement system as shown in FIGS. 1 or 2 may be used.However, if there exist a significant temperature difference between thefirst and second points, the construction of FIG. 1 or FIG. 2 isinadequate since it only takes into account the temperature at the mainUDMS unit. A known reference distance is also required at the reflectionpoint if the temperature at the reflection point is likely to bedifferent to that at the main unit.

FIG. 9 illustrates another preferred embodiment of the presentinvention, in which a known reference distance d3 is provided at thepoint of reflection. The reference distance is formed by using tworeflectors 31 and 32 connected to each other to form of dual reflectorassembly 30. The assembly 30 may be implemented using tri-planereflectors, as illustrated in FIG. 10. The tri-plane reflector iscomprised of three triangular surfaces connected to each other at theedges of the surfaces to form a cone-shaped reflector. Reflectors havingmore than 3 surfaces may also be used. Using the principles of thetri-plane reflector, the apparatus can be compacted by placing thesmaller tri-plane reflector 31 within the larger tri-plane reflector 32.The two reflectors are firmly secured to each other in order to providea fixed reference distance d3. The reference distance is set by thedistance between the apexes of the reflectors (or where the apexes wouldhave been if the existed). Using this arrangement ultrasonic signals arereflected off both reflectors whilst travelling on almost the identicalpath.

A wave incident on the dual reflector system is reflected off theinternal surfaces and returns to the transmitter in such a manner thatthe reflected wave is parallel to the incident wave. The total distancetravelled by the wave, although reflected at different places within thereflector, is constant. The distance travelled inside the reflector istwice the distance from the open face of the reflector to the apex.Consequently, the total distance travelled by a wavefront from thetransmitter to the reflector and back again equals twice the distancebetween the transmitter and the apex of the reflector.

The speed of sound at the dual reflector system is calculated bymeasuring the difference in time taken for signals to be reflected offthe two reflectors. This is done by measuring the time taken for asignal travel between the UDMS unit 1 to the first reflector 31, and fora single to travel between the UDMS unit 1 and the second reflector 32.The difference in these times provides the time taken for a signal totravel between the two reflectors 31 and 32. Since the distance d3between the two reflectors is known, it is then possible to determinethe speed of sound at the reflector system. By averaging the speed ofsound at the subsystem 5 and the reflector system 31, 32, it is possibleto estimate an average speed of sound along the path between the UDMSunit 1 and the reflector system 31, 32.

The timing measurement is performed by examining the receiveddemodulated binary FSK signal and comparing it to a preset thresholdlevel, resulting in a digital output representing 40 kHz or 38 kHz. Thefirst digital transition occurs at the time at which the pulse from thefirst reflector is received. The second digital transition occurs at thetime when the signal reflected by the second reflector is received. Theabsolute accuracy involved with this process is relatively poor due tovariations in transient behaviour for various system components. Howeverwhen dealing with the timing difference between the two reflectedpulses, any variation in absolute accuracy affects the timing of each ofthe two received signals equally. Thus, the measurement of the timedifference for d3 can be made quite accurately. Care should be exercisedto ensure that any temperature gradient present in the measurementenvironment is severe enough to require the implementation of the secondreference subsystem. Otherwise, performance of the UDMS may actually bereduced due to the granularity introduced by the timing measurement ofthe second path. If little or no temperature gradient is present, thenonly the reference subsystem located at the main unit should be used.

An important advantage arises from the existence of two received pulsesinstead of one. A major problem in ultrasonics is the verification ofthe received pulse. It is relatively easy to verify the existence of areceived pulse resulting from the transmitted pulse of the UDMS, purelyfrom the characteristics of the received pulse (ie. it should be easilydistinguishable from other external ultrasonic sources of noise).However, if the transmitted pulse is reflected not only by the placedreflector, but from other objects near the transmission path errors mayoccur. The only practical method is to examine theamplitude/frequency/phase/signal to noise characteristics of thereceived pulse and make a decision as to whether it seems to have beenreflected from the known reflector. Now that two reflectors are present,a much more reliable method of reflected pulse verification may be used.Upon the reception of the first received pulse, it is known that asecond pulse is to be expected within a relatively small time windowafter the first pulse. If a second pulse does not arrive when expected,it is a reasonable indication that the first pulse was from anunexpected source and can be discarded. This is an important advantageover the known systems.

The normal positioning of the reflector when measuring roof height isfor the ultrasonic transducer to be placed on the roof and the reflectoron the ground. The size of the reflector may be varied, but ranges of 50to 120 mm from the apex to the open face may be used.

The reflector, being of a tri-planar design which may be provided with ahole at the apex which would allow dust and small rocks to fall throughthe reflector, thus preventing inaccuracies through contamination of thereflector pair.

FIG. 11 is a block diagram of a preferred embodiment of the electroniccomponents of the UDMS unit of the present invention.

According to the preferred embodiment there is provided a generalpurpose micro-processor 51 which controls the operation of the UDMS unitand processes the distance measurements and temperature measurements todetermine the distance between the UDMS unit and the reflector. Themicro-processor 51 is connected to a transmission circuit 52 whichreceives FSK modulated digital signals from the micro-processor andperforms level shifting and buffering of the digital input signal inorder to deliver a signal of adequate power to the transmittingtransducer 53 for transmission to the reflector. Once the FSK signalsare reflected from the reflector they are received by a receivingtransducer 54 which is connected to a receiver amp 55. The receiver amp55 amplifies the signal receives by the receiving transducer 54. Thereceiver amp 55 is also connected to the micro controller 51, whichcontrols the gain of the receiver amp.

The amplified signal from the receiver amp 55 is sent to a phase lockedloop (PLL) 56 which filters the incoming FSK waveform and produces aclean digitized form which can be fed to the micro controller 51. Thephase locked loop 56 is connected to the micro-processor 51 and providesa digital equivalent of the FSK modulated signal received by thereceiver transducer 54 to the micro-processor 51. The phase locked loop56 is also connected to a level detector 57 and the phase lock loop 56provides a demodulated analog signal which varies between two discreteDC level voltages representing each of the two frequencies used in themodulation. The level detector 57 converts the analog demodulated signalto a digital signal which is communicated to the micro-processor 51.

The receiver amp 55 is also connected to a peak detector 59 whichconverts the signal from the receiver amp 55 to produce an envelope ofthe signal. The peak detector 59 is connected to a threshold detector 58which produces a digital output representing the amplitude of theenvelope of the received signal. The threshold detector 58 warns themicro controller 51 of an incoming signal which is to be analysed.

The preferred embodiment also provides a thermistor 60 which determinesthe temperature and is connected to a temperature signal conditioningunit 61 which provides a digital output of the current temperature tothe micro-processor 51. From this measurement it is possible for themicro-processor 51 to calculate the current speed of sound.Alternatively, the speed of sound may be calculated using the reflectorsystem described above, in which case the thermistor is not required.

The micro controller 51 is connected to an EPROM 67 in which thecontrolling program of the micro-processor 51 is stored. Themicro-processor 51 is connected to a RAM 68 for storing any data used inmeasuring the distance. The micro controller 51 is also connected to aperipheral interface adaptor 62 which is connected to an LCD display fordisplaying a measured distance or to warn of impending collapses. Theperipheral controller 51 mat be provided with a communications line 56for communicating with another computer system. For example, themicro-processor 51 may be under control of a larger computer which isconnected to a number of UDMS units, and the computer can thus instructvarious UDMS units to make measurements of convergence and to processthe information for further use.

The UDMS may be used in a variety of applications. Due to thenon-intrusive nature of the above described ultrasonic measurementsystem, it is possible to employ the system in locations where therewould be considerable risk of damage if conventional extension meterswere used. For example, UDMS can be installed in underground roadways,without disruption to the movement of vehicles. This can be achieved byattaching the unit to the roof (using an existing roof bolt) and placingthe reflector against the rib or side, where it will not be damaged byvehicle or pedestrian traffic. Similarly, the UDMS can be placed tomonitor movement where important equipment is located but is oftenunattended and hence if convergence occurs it would not normally bedetected until a roof fall occurs. Examples include conveyor drives,pumping stations and substations where a roof fall at any of theselocations would cause significant loss of production and expensiveequipment damage. The present invention has particular application inthe mining industry. It may be used in long wall mining situations andin mine openings and access shafts. When used, for example, in an accessshaft, the main unit containing the electronic components andtransducers is positioned on the roof while either a single or dualreflector is placed on the floor.

FIG. 12 illustrates a typical arrangement for installation in a longwall mining application. A long wall shearer (not shown) is used toexcavate a shaft in the long wall mining process. A roof support 40 isused to support the roof of the excavated shaft. The roof support 40 hasa base 41 and a roof supporting member 42 which helps to prevent theroof of the shaft from collapsing. A UDMS unit 43 is attached to thesupporting member 42 and the reflector 44 is attached to the base 41. Insuch an application the UDMS measures the distance or convergence of themember 42 to the base 41, which is virtually the same as the convergentof the roof and floor. By detecting convergence of the member 42 and thebase 41 it is possible to detecting convergence of the shaft roof anfloor which may preceed a collapse. Once convergence is detected theunit 43 can warn the mining operators to take remedial action to preventa total collapse. Due to the small size of the UDMS and reflector, aunit can be installed on each powered roof support, of which there maybe 150 side by side, so that a profile of the convergence across thefull face of the long wall can be monitored with out adversely affectingaccess in an around the long wall system. In such an applicationconvergence monitoring will allow prediction of impending roof problems,and result in significant reduction in lost production due to unexpectedroof movement.

It will be appreciated that features of the above invention may bevaried for different applications. The foregoing description of theembodiments of the invention have been presented for purposes ofillustration only. It is not intended to be exhaustive or to limit theinvention to the embodiments, and many variations and modifications willbe obvious to one skilled in the art.

I claim:
 1. A method of measuring the distance between a first point anda second point in space, comprising the steps of:generating andfrequency modulating an ultrasonic signal using binary frequency shiftkeying, transmitting the modulated signal from the first point towardsthe second point; reflecting the transmitted signal at the second pointback towards the first point; receiving the reflected signal at thefirst point, and phase digitising the received signal by recording datapairs comprising time and place of zero-crossing transitions of thereceived signal, and analysing the recorded transitions in a domainusing only the time and phase data so as to determine the distancebetween the first point and the second point.
 2. A method according toclaim 1, wherein the frequency modulation of the ultrasonic signal usingthe frequency shift keying comprises the step of changing the ultrasonicsignal from a first carrier frequency to a second carrier frequency, ata first point in time.
 3. A method according to claim 2, wherein therecorded transitions are analysed to determine a second point in time atwhich the received signal changes from the first carrier frequency tothe second carrier frequency, thereby providing a time measurement forthe travel time from the first point in space to the second point inspace.
 4. A method according to claim 3, further comprising the step ofdetermining the speed of sound proximate to the first point in space,wherein the speed of sound and the time measurement is used to determinethe distance between the first and second points in space.
 5. A methodaccording to claim 4, wherein the speed of sound is determined along apath between the first and second points in space so as to compensatefor changes in the speed of sound due to changes in temperature at andbetween the first and second points in space.
 6. A method according toclaim 4, wherein the speed of sound proximate the second point in spaceis determined by further reflecting the transmitted signal off a thirdpoint in space which is at a fixed known distance from the second pointin space, receiving both the signals reflected from the second and thethird points in space at the first point in space, and analyising boththe reflected signals to determine the speed of sound proximate thesecond point.
 7. A method according to claim 1, wherein the receivedsignal is further processed to eliminate spurious errors in the receivedsignal, thereby reducing incorrect determinations of the distancebetween the first and the second points in space.
 8. A method ofmeasuring the distance between a first point and a second point inspace, comprising the steps of:generating and frequency modulating anultrasonic signal using binary frequency shift keying by transmitting afirst signal segment having a first frequency followed by a secondsignal segment having a second frequency, where the transition from thefirst frequency to the second frequency occurs at a time t_(o) ;transmitting the modulated signal from the first point towards thesecond point; reflecting the transmitted signal at the second point backtowards the first point; receiving the reflected signal at the firstpoint and digitising the received signal by recording data pairscomprising the time and phase of the received signal at zero-crossingtransitions of the received signal; and analysing the recordedtransitions so as to determine the distance between the first point andthe second point by processing the data pairs of the phase of thereceived signal against time by:(a) finding a first line of best fit fordata pairs corresponding to the first signal segment, (b) finding asecond line of best fit for data pairs corresponding to the secondsignal segment, and (c) finding the point of intersection of the firstand second lines of best fit; whereby the time of the intersection ofthe first and second lines of best fit corresponds to the transitiontime of the ultrasonic signal from time t_(o).
 9. An apparatus formeasuring the distance between a first point and a second point inspace, comprising:an ultrasonic signal generator at the first point forproducing, in use, a frequency modulated ultrasonic signal using binaryfrequency shift keying and transmitting the modulated signal, areflector at the second point capable of reflecting the transmittedsignal, a receiver at the first point capable of receiving the reflectedsignal, signal processing means for phase digitising the received signalby recording data pairs comprising time and place of zero-crossingtransitions of the received signal and for analysing the recordedtransitions in a domain using only the time and phase data so as todetermine the distance between the first point and the second point. 10.An apparatus according to claim 9, wherein the signal processing meansis capable of determining a time measurement for the transmitted signalto travel from the first to the second points in space.
 11. An apparatusaccording to claim 10, further comprising means for determining thespeed of sound proximate the generator, wherein the determined speed ofsound and the time measurement is used to calculate the distance betweenthe first and the second points in space.
 12. An apparatus according toclaim 11, further comprising means for determining the speed of soundproximate the reflector, wherein the determined speed of sound proximatethe reflector and proximate the generator are used with the timemeasurement to calculate the distance between the first and the secondpoints in space.